They are indeed AOT in the original. T2/3, the emitter followers, are selected and matched and their base resistors R5/6 too. 75 -150k.mediatechnology wrote:ricardo - What is the typical value of R5 and R6 in the Schoep's circuit? Are those AOT?
You can take another approach and just use high hfe transistors so there is effectively no voltage across the resistors so Vcb = 0. The emitter followers will easily swing to Vce < 0.1V so you have -0.5 Vp for each leg. The +ve swing depends on the current draw of the mike. A total current of 5mA (so as not to upset some vintage N***s) gives 2.5mA x 6k8 = 17V across the P48V resistors. But the 6k8 drives half the 2k preamp load so 17 x 1k/(6k8 + 1k) = 2.18Vp.
Though I've played a lot with Schoeps circuits, I've never designed one into a commercial product. In the early 80's, there was on old DIN P48V 0.5mA standard that only a lil' ol' mikemaker in Hebden bridge met. None of the big Germans did .. except for the KM84 which was the original P48V mike. We did it with transformers and got good overload with a sneaky Class B circuit.
There are many advantages with transformer output, the most obvious one being RFI/EMI design. A more subtle one is that you can arrange it so the mike can drive unbalanced inputs without loss of signal or noise degradation (You obviously lose the CMRR of a balanced input) I used Calrec transformer stick mikes into a souped up recorder's unbalanced input and later directly into a Sony PCM-F1. I made up a box with Sowter transformers for the F1 but it was another box to carry around.
For a deluxe mike to be hand crafted by virgins from Unobtainium (like genuine Schoeps), I've drawn up a chart for R5/6 vs hfe of individual T2/3 so the -ve swing on each leg will match the above 2.18Vp +ve swing
hfe 196 218 240 262 295 327 360 393 425 469 513 556 611 676 742
Rb 180 200 220 240 270 300 330 360 390 430 470 510 560 620 680k
Of course dis beach bum is just after level and leaves THD & other niceties to da pedants